The present invention relates to frequency acquisition in general and to frequency acquisition in the presence of high power adjacent channels, in particular.
Reference is now made to FIGS. 1A and 1B. FIG. 1A is a schematic illustration of frequency versus power, describing the initial stage of a initial frequency synchronization procedure, known in the art. The present example describes a closed loop automatic frequency control (AFC) mechanism.
FIG. 1B is a schematic illustration of frequency versus power, describing the final stage of the initial frequency synchronization procedure of FIG. 1A.
Arrow 14 represents the frequency of a mobile unit which detects and attempts to lock and synchronize with the carrier frequency 10 of a base unit transmitter having a value of FBASE, which is located near by. In the present example the mobile unit further detects a carrier frequency 12 provided by a neighbor transmitter, having a value of FNEIGHBOR. The value of the mobile unit F0MOBILE is located between the values of the base unit frequency FBASE and the neighbor mobile transmitter frequency FNEIGHBOR.
In the present example the mobile unit 14 detects the signals provided by base 10 and the neighbor 12 wherein the received power of the neighbor 12 is higher than the received power of the base unit 10.
According to conventional initial synchronization procedures, the mobile unit frequency is synchronized with the frequency having the highest received power, which in the present example is the neighbor frequency 12.
It will be noted that often the received frequencies are filtered so as to exclude undesired signals. Such a filter is represented by arc 16. These techniques often fail when the power of the undesired signal is significantly high.
Accordingly the synchronization mechanism of the mobile unit sets synchronization path towards the neighbor frequency FNEIGHBOR and starts progressing its frequency 14 towards FNEIGHBOR. Finally the synchronization mechanism allows the frequency of the mobile unit 14 to acquire and synchronize with the frequency of the neighbor unit 12. This is shown in FIG. 1B by aligning line 12 and arrow 14. As can be seen, at this stage the frequency 10 of the base transmitter is filtered out by the filter 16.
A conventional synchronization mechanism provides frequency shifts within a limited range, determined by its structure, such as VCO voltage and the like. It will be appreciated by those skilled in the art that the FNEIGHBOR can be located outside this range in such a case, FMOBILE, might get stuck at the boundary frequency value which is closest to FNEIGHBOR.
It will be appreciated by those skilled in the art that such situations, where the frequency of the mobile unit 14 is synchronized with the frequency of neighbor unit 12 instead of the frequency of the base unit 10, is not acceptable.
It is an object of the present invention to provide a novel method for performing accurate initial frequency acquisition in the presence of high power adjacent channels.
It is a further object of the present invention to provide a novel device for performing accurate initial frequency acquisition in the presence of high power adjacent channels.
In accordance with the present invention there is thus provided a method for acquiring frequency of a desired channel having a carrier frequency FMAIN, for a dynamic receiver frequency FMOBILE, from a starting frequency FSTART, in the presence of high power adjacent interfering channels.
The starting frequency FSTART is shifted from FMAIN by not more than a predetermined frequency gap xcex94F. The method includes the steps of:
determining a first frequency boundary and a second frequency boundary;
detecting channels within a filtering bandwidth;
selecting a dominant channel from the detected channels;
progressing the dynamic receiver frequency FMOBILE towards the carrier frequency of the dominant channel;
detecting when the step of progressing has exceeded one of the first frequency boundary and the second frequency boundary;
restarting the step of detecting channels, from the other of the one of the first frequency boundary and the second frequency boundary; and
repeating from the step of detecting channels.
According to another aspect of the present invention, one of the first frequency boundary and the second frequency boundary is FSTARTxe2x88x92xcex94F, while the other is FSTART+xcex94F.
The method of the invention can also include the step of determining a frequency advance direction. The frequency advance direction can be fixed at the beginning of each frequency acquisition cycle, wherein the frequency acquisition cycle is determined from the point where FMOBILE shifts from FSTART until the point where FMOBILE returns to FSTART.
The step of progressing can be performed in a frequency step FSTEP. The value of the frequency step FSTEP can be infinitesimal with comparison to the predetermined frequency gap xcex94F, or adjustable. Accordingly, the method can further include the step of adjusting the frequency step FSTEP after each step of detecting channels.
In accordance with another aspect of the present invention, there is provided a device for acquiring frequency of a desired channel having a carrier frequency FMAIN, for a dynamic receiver frequency FMOBILE, from a starting frequency FSTART, in the presence of high power adjacent interfering channels.
The device is connected to an antenna via a receiver and to a reference frequency FREFERENCE source. The device includes controllable frequency generating means for generating an internal frequency FINTERNAL, frequency shift means connected to the controllable frequency generating means, and to the receiver, for shifting received frequency FRECEIVED, of a received channel, according to the internal frequency FINTERNAL.
The device also includes a frequency shift detector, connected to the frequency shift means, for detecting a frequency difference between the internal frequency FINTERNAL and the received frequency FRECEIVED, with respect to the reference frequency FREFERENCE, thereby producing a frequency shift value FSHIFT.
The device further includes loop filtering means, connected to the frequency shift detector, for filtering the frequency shift value FSHIFT, thereby producing a filtered frequency shift value FSHIFT-FILTERED, and controlling means, connected to the controllable frequency generating means and to the loop filtering means, for determining a frequency step FSTEP from the filtered frequency shift value FSHIFT-FILTERED.
The controlling means provide the frequency shift value FSHIFT to the controllable frequency generating means. The controllable frequency generating means adjust the internal frequency FINTERNAL according to the frequency shift value FSHIFT, and the controlling means control the controllable frequency generating means to generate frequency in a range from a first frequency boundary FFIRST and a second frequency boundary FSECOND.
According to one aspect of the invention, the controlling means set the frequency shift value FSHIFT to be FSECONDxe2x88x92FINTERNAL, when |FINTERNALxe2x88x92FSTART|xe2x89xa7|FINTERNALxe2x88x92FFIRST|, while the controlling means set the frequency shift value FSHIFT to be FFIRSTxe2x88x92FINTERNAL, when |FINTERNALxe2x88x92FSTART|xe2x89xa7|FINTERNALxe2x88x92FSECOND|.
According to another aspect of the invention, the device further includes frequency filtering means, connected between the frequency shift detector and frequency shift means.
The controlling means reset the loop filtering means when setting the frequency shift value FSHIFT to be FSECONDxe2x88x92FINTERNAL or FFIRSTxe2x88x92FINTERNAL.